Sibling cells undergo mitosis at about the same age, but pairs of distantly related cells do not. This suggests that at the time of their formation, both sibling cells inherit something from their mother that predisposes them to divide at a particular age. The objectives of this proposal are to characterize the inherited structures or processes that cause sibling cell generation times to be correlated and to test specific hypotheses concerning the agent responsible for the sibling correlations. Mitotic tissue culture cells will be seeded onto substrates that are covered by gold particles. The pair of cells derived from the mitosis can be identified as siblings on the basis of the connected tracks that they make in the gold as they move away from one another. A computer-based image analysis system will be used not only to identify large numbers of sibling cells but also to acquire quantitative physiological and morphological data about the sibling pairs. The fraction of sibling pairs exhibiting zero, one, or two members that have passed several cell cycle landmarks will be determined as a function of cell age. These data will indicate which cell cycle phases are most strongly correlated between sibling cells (the phases during which the inherited property acts) and will also be used in quantitative tests of the transition probability cell cycle model, which attributes the correlations to a process that begins in mother cells and terminates in offspring cells. Related serum shiftdown/shiftup experiments will measure the stability of the inherited property during aqiescence, as well as the extent to which the sibling generation time correlations are influenced by hormone levels. Comparison of the similarity of sibling cell tracks with the similarity of sibling cell cycle events will test the hypothesis that generation-time correlations are due to initialy morror-image cytoskeletons, as would be expected from a model which proposes that determinants of cell shape control the cell cycle. Comparison of sibling nuclear structures by quantitative scanning electron microscopy will test the hypothesis that sibling generation time correlations are due to the inheritance of equivalent sibling nucleolar organizing centers.